Advancements in material fabrication demand the tailoring of their properties to respond to immediate and future technical challenges. This implies creating materials that may not even exist naturally, or modifying existing ones to promote suitable behavior. Therefore, to construct such materials, one needs to characterize them to understand their electronic, magnetic and chemical behavior. As compared to the bulk state, materials behave differently at the nanoscale. Therefore, a tool that is capable of providing contrast, that is, information on chemical, electronic and magnetic features of nanostructures will constitute an important technique that allows meeting the challenges of the 21st century.
Scanning probe microscopes (hereinafter “SPM”) such as the scanning tunneling microscope (hereinafter “STM”) or other variants can achieve the required spatial resolution.
However, one of the major problems with SPM's is that direct chemical or magnetic contrast cannot be obtained. This is due to the fact that tunneling electrons originate only from states close to Fermi energy, i.e., those states that do not carry information specific to the material under investigation. On the other hand, X-ray spectroscopies allow determining chemical and magnetic properties of materials, albeit with limited spatial resolution.
Since its development, STM has shown unprecedented power in resolving surface atomic and electronic structure with the ultimate spatial resolution. Recent developments on spin-polarized STM have also allowed the study of magnetism down to single atomic scale. As noted hereinbefore, elemental sensitivity has been lacking due to the fact that STM only probes the valence and conduction band electrons that are located near the Fermi level. Unlike core-level electrons, the electronic structure near the Fermi level does not carry direct information about the atomic species, and therefore cannot be used to “fingerprint” chemical elements on the surface.
In order to achieve elemental sensitivity at the ultimate spatial resolution, synchrotron X-ray STM (hereinafter “SXSTM”) and synchrotron X-ray atomic force microcopy (SXAFM) have been developed and show great analytical potential. In SXSTM, a synchrotron X-ray beam illuminates onto the sample surface during scanning, resulting in photo-excitations that can either enhance or suppress the tunneling process depending on the bias polarity. Because the excitation energies are element specific, at any particular X-ray energy, the intensity of these excitations will vary locally on the sample's surface depending on the local chemical composition. Therefore, as the tip is scanning, the local variation in photo-excitations results in additional modulations in the tunneling current. By scanning with different X-ray energies, one can then achieve complete chemical mapping of the surface. Magnetic contrast can also be obtained when the polarization of the X-rays is utilized. One serious problem SXSTM suffers from so far is the interference of the X-ray excitations on the STM feedback system. Since the X-ray induced currents are superimposed onto the conventional tunneling currents, the topography and the chemical contrasts are convoluted together. More problematically, X-ray excitations can destabilize the feedback, sometimes causing the tip to fully retract.